Fix quadratic ReDoS in PEM import block scanning

Replace the backtracking _PEM_BLOCK regex with a forward-only scanner
that indexes END markers by label, so repeated unterminated headers do
not cause the regex to rescan the remaining input.

Impact: 80 KiB payload went from 8.34 s to 0.001 s.

server/packages/kca-core/tests/test_core.py

@@ -1,6 +1,7 @@
 """Core domain tests: names, extensions, keys, certificates, CRLs and exports."""
 from __future__ import annotations
 
+import time
 import unittest
 from datetime import datetime, timedelta, timezone
 
@@ -273,6 +274,30 @@ class ExportImportTest(unittest.TestCase):
         items = imports.parse_file(pem.data, "export.crt")
         self.assertEqual(items[0].kind, "certificate")
 
+    def test_pem_bundle_ignores_incomplete_blocks(self) -> None:
+        certificate = exports.export_certificate(self.cert, "pem", name="export").data
+        private_key = keys.private_pem(self.key)
+        pem = (
+            b"before\n-----BEGIN INCOMPLETE-----\nignored\n"
+            + certificate
+            + b"\n"
+            + private_key
+            + b"\nafter\n"
+        )
+        items = imports.parse_pem_bundle(pem)
+        self.assertEqual([item.kind for item in items], ["certificate", "key"])
+        mismatched = b"-----BEGIN CERTIFICATE-----\nignored\n-----END PRIVATE KEY-----"
+        self.assertEqual(imports.parse_pem_bundle(mismatched), [])
+
+    def test_unterminated_pem_headers_complete_quickly(self) -> None:
+        header = b"-----BEGIN UNTERMINATED-----\n"
+        data = header * ((80 * 1024) // len(header))
+        started = time.perf_counter()
+        items = imports.parse_pem_bundle(data)
+        elapsed = time.perf_counter() - started
+        self.assertEqual(items, [])
+        self.assertLess(elapsed, 2.0, f"PEM scanning took {elapsed:.3f}s for {len(data)} bytes")
+
     def test_key_exports(self) -> None:
         for fmt in ("pem-public", "pem-private", "pkcs8", "ssh-public", "jwk-public", "pvk"):
             result = exports.export_key(self.key, fmt, name="export")